Human body communication is a technology using a conductive human body as a communication channel, which comprises: transmitting information to an electrode of a transmitter that is coupled to one side of the human body, receiving the transmitted information in a receiver through an electrode of the receiver that is coupled to another side of the human body or arranged outside the human body, and recovering the transmitted information.
Such human body communication technologies have been used for communications between a variety of portable devices such as a personal digital assistant (PDA), a portable personal computer, a digital camera, an MP3 player and a mobile phone, and communications with fixed devices, for example communications with a printer, a credit card payment system, a TV receiver system, an entry control system, and bus and subway fare payment systems, etc.
Human body serves as an antenna within wide frequency spectrums since the human body is composed of extraordinarily various substances and shapes and has low conductivity and high permittivity. Therefore, the human body such characteristics can be used as an antenna in communications. However, the communications may become unstable since undesired noise signals may be induced from other adjacent users or external electronic equipment, and also unstable due to the presence of surrounding objects and the distance between devices, and the position of the devices. As a result, human body communication technologies have developed to support stable communications without any adverse effects of this interference.
As one of conventional human body communication technologies, a system for directly transmitting a digital signal requires a very wide frequency band. In this case, when characteristics of a received signal are recovered, it is confirmed that strong interferences induced from peripheral equipment are also received in a receiver in addition to the signals transmitted from a transmitter. Where there are interferences generated within a transmission band, a receiver may not properly separate a desired signal although the receiver has highly excellent sensitivity. Also, a more than several tens of MHz bandwidth is required to send digital signals having several Mbps. In this case, when these digital signals are applied to a human body, the signals having a frequency over a certain frequency band are emitted from the human body. Therefore, it is difficult to construct a stable network due to the presence of the interferences between many users even when the users do not come in contact with each other.
In order for many adjacent users to stably communicate with each other without any interference between the users, there has been proposed a method of reducing an occupied frequency of signals transmitted through a human body to a frequency range at which the signals transmitted through the human body do not interfere with those from adjacent users. However, the proposed method has problems in that a transmission speed of signal may be significantly reduced due to the restricted usable frequency resource, and signals with at least several tens of MHz bandwidth are emitted without being defined in the human body when the signals are applied to a human body in consideration of the occupied frequency bandwidth, which causes interferences between adjacent users.
In order to solve the above problem, human body communication methods, such as scrambling, channel coding, interleaving and spreading methods, using a restricted pass band spanning from 5 MHz to 40 MHz bandwidth and employing information on user's innate identity (ID) have been proposed.
However, a pass band having an intermediate frequency (fc) that is used in most of communication systems have been used as the limited frequency band, and therefore the communication systems requires additional analog transmitter and receiver terminal such as a digital-analog converter, an analog-digital converter, an intermediate frequency converter and the like, which indicate that these communication systems are disadvantageous in the term of low power consumption. Furthermore, the conventional human body communication technologies have problems in that they may not apply to the field of a variety of applications requiring convenience and non-contact communications since they use a signal electrode system that always comes in contact with a human body to transmit data in the form of electric filed.